This invention relates in general to photodetectors and, in particular, to an improved photodetector and a device employing the photodetector for converting an optical signal into an electrical signal.
CD-ROM and DVD-ROM drives have become increasingly important and popular for use with personal computers and amusement game machines. CD-ROM and DVD-ROM drives typically employ optical pickup usually comprising two tracking photodetectors and four high speed split photodetectors, such as that explained and shown in the article xe2x80x9cHigh Speed SI-OEIC (OPIC) For Optical Pickup,xe2x80x9d by Takimoto et al., IEEE Transactions on Consumer Electronics, Vol. 44, No. Feb. 1, 1998, pages 137-142.
As noted by the Takimoto article, CD-ROM and DVD-ROM drives are progressing in terms of system compactness and performance. But when the size of these drives are reduced, cross-talk has become an issue due to the close spacing between the components of the drive. To reduce the influence of external noise from the motor and other electrical circuits, the photodiode used in optical pickup has been integrated with the preamplifier circuit that is used to amplify the output of the photodetector.
CD-ROM and DVD-ROM drives have increasingly been used for reading video data, such as data for motion pictures. For such applications and for improved performance in other applications, it is important for the CD-ROM and DVD-ROM drives to have wide bandwidths.
Optical pickup components currently employed in CD-ROM and DVD-ROM drives are typically bipolar devices. While bipolar devices may have acceptable performance for such applications, with the intense competition in the consumer electronics industry, it is desirable to provide alternative designs that are cheaper than the current optical pickup designs.
None of the above-described conventional optical pickup devices for CD-ROM and DVD-ROM drives are entirely satisfactory. It is, therefore, desirable to provide an improved optical pickup and photodiode design to avoid some of the difficulties described above.
Applicant proposes an optical photodetector device implemented as CMOS devices which are much cheaper than bipolar photodetector devices. Preferably, the CMOS processing circuit for amplifying the photodiode output and the photodiode may be implemented in the same semiconductor substrate.
In another aspect of the invention, the P or N type semiconductor material that forms one side of the PN junction has at least two portions that are spaced apart by not more than twice the one-sided junctioned depletion width in a configuration referred to herein as a distributed structure or configuration. By employing a PN junction of such type where the semiconductor material forming one side of the junction is so distributed, this has the effect of increasing both the density and amplitude of electric field in the depletion region of the photodiode, thereby reducing drift time of carriers in the depletion region. The responsivity can be further increased by preferably employing a highly doped semiconductor region between the two portions of the semiconductor material that forms one side of the junction.
When the photodiode with a distributed configuration of semiconductor material as one side of the PN junction is employed in a CD-ROM or a DVD-ROM drive, it is preferable for the two spaced apart portions of such material to be spaced apart by a spacing in the range of 5 to 15 microns.
To further enhance the responsivity of the photodiode, an anti-reflective filter is employed over the entire photosensitive region on the surface of a semiconductor substrate. The filter includes a first layer of silicon nitride and a second dielectric layer contiguous with the first layer.
In fabricating CMOS devices, a threshold Vth implantation (hereinafter referred to as xe2x80x9cVth implantxe2x80x9d or xe2x80x9cVth implantationxe2x80x9d) is performed to adjust the threshold voltage(s) of the CMOS devices. Where the CMOS devices and the photodetector are fabricated on the same substrate, a mask is preferably provided to shield the portion of the substrate in which the photodetector region has been or is to be formed during the implantation, and the Vth implantation is performed only on the portion of the substrate that is not shielded by the mask, so that such implantation does not affect the photodetector.